Nuclear reactors need to be periodically maintained and inspected as well as refueled. During refueling operations, refueling of the core and related fuel operations are performed, as well as on-site inspection and maintenance within the reactor vessel of reactor pressure vessel internals which are otherwise inaccessible during reactor operations.
Several boiling water reactors have experienced cracks or defects resulting from radiation assisted stress corrosion cracking and intergranular stress corrosion cracking in the reactor pressure vessel top guide structure which serves to align and maintain the upper portion of the fuel assemblies in proper position.
In the past, remote visual examination has been used to locate and characterize these defects. Due to both the construction of the top guide structure which is comprised of two sets of straight parallel grid members which are interconnected to form a lattice of rectangular shaped openings, and the position of the upper ends of the fuel assemblies within these openings, many areas of the top grid structure cannot be accessed or cannot be adequately inspected by visual examination. Areas of the grid members which intersect one another or are underneath one or the other cannot be inspected by visual examination. When cracking or defects occur, the grid members obscure from view the resultant cracks or defects. Areas of the top guide structure which are inaccessible to visual examination have frequently gone uninspected. Even in those limited areas of the top guide structure which can be seen, visual inspection is inherently inadequate to identify defects which are too small to be visually detected. Furthermore, visual examination of even those portions of the top guide structure which can be seen, cannot adequately characterize the depth of a defect which is of equal if not greater importance than identifying the existence of a defect. Even for those areas which can be visually inspected and defects identified, visual inspection cannot be relied upon to provide accurate defect sizing.
Recording the results of visual examination of the plethora of grid members of the top guide structure can be, and frequently is, subject to error due to the difficulty of correlating the presence and location of a defect in a particular grid member to a frame of reference of all the grid members of the top guide structure. Since inspection of the top guide structure has to be performed remotely and underwater, errors due to parallax can significantly impair the accuracy of the results.
In order to inspect even those portions of the top guide structure which can be visually inspected (other than the top edge of the grid members), the nuclear fuel assemblies which are positioned in the openings in the top guide structure have had to be removed. Accordingly, all or at least some of the nuclear fuel assemblies have had to be at least lifted from the reactor core and probably removed from the reactor pressure vessel in order to permit the performance of even a limited visual inspection. In addition, in order to perform visual inspections, the fuel handling machine needs to be continuously available rendering it effectively unavailable for refueling operations or for other purposes even after the fuel assemblies have been removed from the reactor vessel. The need to first remove some or all of the nuclear fuel assemblies from the reactor core or reactor pressure vessel to perform an inspection of the top guide structure involves several days of critical inspection time which can be very costly because of increased refueling outage time.
It would therefore be an advantage and is an object of the present invention to provide a ultrasonic inspection system and method and for inspecting the top guide structure in boiling water reactor pressure vessels for cracks or defects which avoids or overcomes the above-stated disadvantages and which does so remotely, more effectively and accurately, and without the necessity to remove any of the fuel assemblies from the reactor core.